1. Field of the Invention:
The present invention relates to a method of strengthening low carbon steel cold rolled strip and sheet stock and articles formed therefrom. In particular, a preferred embodiment of this invention involves the concept of producing stamped or deep drawn parts from a low strength, deep drawing-quality steel, and subsequently strengthening the parts by alloy-nitrogen precipitation strengthening. In another embodiment cold rolled and annealed sheet and strip stock can be strengthened by alloy-nitrogen precipitation strengthening before forming to obtain higher yield strength than has hitherto been available in low carbon steel while at the same time producing very high average plastic strain ratios, r.sub.m values.
Prior art high strength, cold rolled sheet stock has exhibited r.sub.m values of about 1.0 to 1.2. The present invention attains very high plastic strain ratios (r.sub.m values ranging from about 1.8 to about 2.2), together with yield strengths up to 90 ksi or higher.
2. Description of the Prior Art:
U.S. Pat. No. 3,847,682, issued Nov. 12, 1974 to Rollin E. Hook, discloses a method of increasing the yield strength of a low carbon steel sheet stock or an article formed therefrom by heating in an atmosphere comprising ammonia and hydrogen. A deoxidized, low carbon steel containing from about 0.002% to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, a nitrogen-forming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02% to about 0.2%, columbium in solution is from about 0.025% to about 0.3%, and zirconium in solution is from about 0.025% to about 0.3%, and balance essentially iron, is heat treated at 1100.degree. to 1350.degree. F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride, Within a preferred temperature range of 1100.degree. to 1300.degree. F the preferred furnace atmosphere comprises ammonia-hydrogen mixtures having 3% to 6% by volume ammonia. The maximum ammonia concentrations which can be used within this temperature range and which avoid the formation of an iron nitride surface layer are as follows:
1100.degree. F -- about 10% ammonia, PA1 1200.degree. F -- about 6% ammonia, PA1 1300.degree. F -- about 3% ammonia.
This patent further discloses that nitrogen taken into solid solution as a result of the alloy-nitrogen precipitation strengthening step can present weldability problems and can result in high ductile-to-brittle Charpy impact transition temperatures. However, if the nitriding step is followed by a denitriding step which involves annealing in pure hydrogen at about 1200.degree. F for at least two hours, the excess nitrogen is removed (with only a 10-20% reduction in yield strength), thereby eliminating welding porosity and substantially reducing the ductile-to-brittle transition temperature with consequent improvement in Charpy impact energy values.
Case hardening by heat treating in an ammonia containing atmosphere to form an iron-nitrogen austenitic structure which is transformed by quenching to a martensitic structure having high surface hardness, has been practiced for many years, and a typical process relating to nitriding of a "Nitralloy" type steel is disclosed in U.S. Pat. No. 3,399,085, issued Aug. 27, 1968 to H. E. Knechtel et al.
U.S. Pat. No. 3,215,567, issued Nov. 2, 1965 to H. Yoshida; U.S. Pat. No. 3,281,286, issued Oct. 25, 1966 to M. Shimizu et al; and U.S. Pat. No. 3,303,060, issued Feb. 7, 1967 to M. Shimizu et al, disclose denitriding and decarburizing of cold-rolled low carbon steel sheet and strip stock. When denitriding is desired in these prior art processes, an atmosphere containing in excess of 70% by volume hydrogen is used (derived from AX or HNX gas) at temperatures ranging from about 600.degree. to about 750.degree. C.
While the above-mentioned U.S. Pat. No. 3,847,682 thus provides a method of strengthening low carbon steel strip or sheet, or articles formed therefrom by stamping or deep drawing, for optimum weldability and Charpy impact properties, the product requires denitriding after the alloy-nitrogen precipitation strengthening step. According to the prior art denitriding must be conducted in an atmosphere containing more than about 70% hydrogen at temperatures of about 600.degree. to about 750.degree. C, for periods of time ranging up to 40 hours. It is thus evident that such a denitriding step is expensive, commercially impractical and dangerous by reason of the high hydrogen content of the annealing atmosphere.